Field of the Invention
The present invention relates to an active imager for obtaining information of an object by using electromagnetic waves. In particular, the present invention relates to an active imager for obtaining an object image (image information) by using electromagnetic waves in frequency regions from a millimeter wave band to a terahertz wave band (30 GHz to 30 THz) (hereinafter also referred to as THz radiations).
Description of the Related Art
In astronomy, because direct THz radiations to be imaged are extremely weak, the high sensitivity of the sensors used for imaging is obtained by cooling the sensor at cryogenic temperatures. In terrestrial applications, cryogenic cooling being cumbersome for many applications, the sensitivity of the sensors becomes insufficient for direct imaging. As a result, the scene or sample to be imaged must be illuminated by THz radiations while the transmitted or reflected radiations, depending on the configuration of the system, are acquired by the imager. The imager may contain a single sensor or probe scanning a surface or an array of sensors and switches. When this array is part of a camera which utilizes a focal lens, it is known as a focal plane array (FPA).
The sensitivity of terahertz sensors operating at room temperature is too low to achieve direct imaging in most applications. The power per unit frequency of radiations emitted by a blackbody at 300 K, 1 THz, for a 1 millimeter square surface and 1 steradian is roughly 10−19 W/Hz. Supposing the sensor being filtered with a frequency band of 100 GHz, the energy reaching one pixel is roughly 10 pW. Today's best sensors in the THz range and operating at room temperature possess a noise equivalent power (NEP) not better than 4 pW/√Hz. Using a scanning frequency band of 1 kHz, they achieve a signal-to-noise ratio of 1 when the input power is 126 pW. When measuring a blackbody at 1 THz with the above conditions, the signal-to-noise ratio is 8×10−2. This signal-to-noise ratio is too low to extract any signal from the background noise in most applications. In order to increase the signal-to-noise ratio, it is therefore necessary to illuminate the scene or sample to be imaged with terahertz radiations. In other words, it is necessary to achieve active imaging. This situation is similar to optical photography, in which the scene is either illuminated by the light of the sun or an artificial light, for example a flash, or a combination of both.
U.S. Pat. No. 7,884,942 of Tomoyuki Umetsu discloses a probe apparatus and terahertz spectrometer. The probe consists of two photoconducting pairs of electrodes integrated on a substrate and two lenses on the opposite side of the substrate, each one facing, through the substrate, one of the two pairs of electrodes. A laser beam is focused on each pair of electrodes. One pair of electrodes is used for emission while the other one is used for detection. As the laser light impinges on the emitting pair of electrodes, THz radiations are emitted in the substrate and propagate into the lens. Thanks to the geometry of the lens, these radiations are focused on a particular position on a sample facing the lens. The THz radiations reflected by the sample are then collimated by the lens facing the detecting pair of electrodes and propagate through the substrate until the detecting pair of electrodes. When the laser beam impinges on the detecting pair of electrodes, the resistivity between the electrodes is lowered and an electric signal can be recorded by the electrodes. Due to a particular angle of the axis of both lenses, the position on the sample on which the emitted THz radiations from the emitting lens impinges corresponds to the position on the sample by which the reflected THz radiations are collimated by the detecting lens. The probe can be scanned along a surface in order to produce an image. However, mechanically scanning a variety of positions requires a large amount of time and cannot lead to high frame rates. Alternatively, an array of such pair of emitting and detecting electrodes and lenses is provided by the application in order to produce an image without resorting to a mechanical scanning and therefore in order to achieve a higher frame rate than in the case of mechanical scanning. In this application, in order to condensate the radiations propagating through the substrate, one lens is needed for each pair of electrodes. In order to be effective, the diameter of the lens must be larger than several wavelength of radiation collimated by the lens according to the calculation of the Airy disk. For example, if radiations are emitted at 1 THz, then the wavelength of the radiations is 300 μm, and the diameter of the lens must be more than 1 mm in order for the lens to be effective as a focusing element. As two lenses are needed for each pair of emitting and detecting electrodes, the distance between two sensing elements, or in other words the distance between two consecutive pixels, is also several wavelengths. As a result, the lateral resolution of this imager is limited to several times the imaging wavelengths in order to be efficient.
M. B. Johnson et al. (M. B., Johnston, et al. Generation of high-power terahertz pulses in a prism. s.l.: Optics Letters, 2002. 27(21)) have reported on the generation of THz pulses in a prism. In this reference, THz radiations are generated applying a laser pulse on an InAs epilayer placed on a bulk GaAs prism and the mechanism which produces the THz radiations is that of the photo-Dember effect. The presence of the prism is responsible for a particular orientation of the dipole generated by the laser in the InAs epilayer, leading to a high power for the terahertz radiations. Depending on the applications, collimation of the THz radiations by a focusing element, such as a lens, may not be necessary, even if the authors state that collection optics leads to higher powers of the THz radiations. Again, one limitation of this reference is the size of the optional collection optics which diameter should be more than several wavelengths of the emitted THz radiations. Another limitation is the width of the GaAs prism which is reported by the authors to be more than 700 μm whereas the wavelength of the emitted radiations is roughly 300 μm. Finally, the geometrical arrangement of the system is another of its limitations. A laser beam must be directed at 45° on the same surface that emits the THz radiations. As a consequence, the emitting surface of the system must by cleared off and the system cannot be used for imaging samples which are in close distance to it or in contact to it.
U.S. Pat. No. 7,689,070 of Toshihiko Ouchi discloses on a high frequency electrical signal control device and sensing system. In this reference, a first laser beam impinges on an emitting photoconductive electrode, and a second laser beam impinges on a detecting photoconductive electrode. The emitted THz radiations and the THz radiations used for detection are transmitted from the electrodes to a single antenna using electrical connections or waveguides. The emitted and detected THz radiations propagate perpendicularly to the surface of the substrate and not in the substrate. As a result, no focusing element such as a lens is needed. However, because the emitted and detected radiations use the same antenna, they need to be separated in the circuitry. This operation can be performed by a delay line, and especially a mechanical delay line. As a result, the operation of this delay line prevents the realization of instant imaging.
The disclosures of the previous references are capable of providing active imaging in the THz range. However, some are limited in the lateral resolution which can be obtained. Some others are limited to samples or scenes which are far away from the sample, or they are limited to slow response because of the presence of delay lines.